Phylogenomic approaches to common problems encountered in the analysis of low copy repeats: the sulfotransferase 1A gene family example

Abstract

Background: Blocks of duplicated genomic DNA sequence longer than 1000 base pairs are known as low copy repeats (LCRs). Identified by their sequence similarity, LCRs are abundant in the human genome, and are interesting because they may represent recent adaptive events, or potential future adaptive opportunities within the human lineage. Sequence analysis tools are needed, however, to decide whether these interpretations are likely, whether a particular set of LCRs represents nearly neutral drift creating junk DNA, or whether the appearance of LCRs reflects assembly error. Here we investigate an LCR family containing the sulfotransferase (SULT) 1A genes involved in drug metabolism, cancer, hormone regulation, and neurotransmitter biology as a first step for defining the problems that those tools must manage.

Results: Sequence analysis here identified a fourth sulfotransferase gene, which may be transcriptionally active, located on human chromosome 16. Four regions of genomic sequence containing the four human SULT1A paralogs defined a new LCR family. The stem hominoid SULT1A progenitor locus was identified by comparative genomics involving complete human and rodent genomes, and a draft chimpanzee genome. SULT1A expansion in hominoid genomes was followed by positive selection acting on specific protein sites. This episode of adaptive evolution appears to be responsible for the dopamine sulfonation function of some SULT enzymes. Each of the conclusions that this bioinformatic analysis generated using data that has uncertain reliability (such as that from the chimpanzee genome sequencing project) has been confirmed experimentally or by a "finished" chromosome 16 assembly, both of which were published after the submission of this manuscript.

Conclusion: SULT1A genes expanded from one to four copies in hominoids during intra-chromosomal LCR duplications, including (apparently) one after the divergence of chimpanzees and humans. Thus, LCRs may provide a means for amplifying genes (and other genetic elements) that are adaptively useful. Being located on and among LCRs, however, could make the human SULT1A genes susceptible to further duplications or deletions resulting in 'genomic diseases' for some individuals. Pharmacogenomic studies of SULT1Asingle nucleotide polymorphisms, therefore, should also consider examining SULT1A copy number variability when searching for genotype-phenotype associations. The latest duplication is, however, only a substantiated hypothesis; an alternative explanation, disfavored by the majority of evidence, is that the duplication is an artifact of incorrect genome assembly.

Figure 1

Genomic organization of the SULT1A LCR family. (A) 30 LCRs (red) aligned to the SULT1A3 LCR (blue). Core sequences of SULT1A and LCR16a families are shown between dashed lines. (B) Chromosome 16 positions of 29 SULT1A3-related LCRs. (C) Known genes, bacterial sequencing contigs, and LCRs (outlined in boxes) in three 350 kbp regions of chromosome 16.

Figure 2

SULT1A gene tree. TREx upper-limit date estimates of hominoid SULT1A duplications are shown as Ma in red. K_A/K_S values estimated by PAML are shown above branches. Infinity (8) indicates a non-reliable K_A/K_S value greater than 100. The 1A3/1A4 branch is dashed. NCBI accession numbers of sequences used: chimpanzee 1A1 [Genbank:BK004887], chimpanzee 1A2 [Genbank:BK004888], chimpanzee 1A3 [Genbank:BK004889], ox [Genbank:U34753], dog [Genbank:AY069922], gorilla 1A1 [Genbank:BK004890], gorilla 1A2 [Genbank:BK004891], gorilla 1A3 [Genbank:BK004892], human 1A1 [Genbank:L19999], human 1A2 [Genbank:U34804], human 1A3 [Genbank:L25275], human 1A4 [Genbank:BK004132], macaque [Genbank:D85514], mouse [Genbank:L02331], pig [Genbank:AY193893], platypus [Genbank:AY044182], rabbit [Genbank:AF360872], rat [Genbank:X52883].

Figure 3

Synteny plots demonstrating SULT1A3 is the progenitor locus of the hominoid SULT1A family. Each box shows a VISTA percent identity plot between a section of the human genome and a section of a rodent genome. Different rodent genomes and alignment methods are indicated as follows: 1 = mouse (Oct. 2003 build) multiple alignment method (MLAGAN); 2 = rat (June 2003 build) multiple alignment method (MLAGAN); 3 = mouse (October 2003 build) pairwise alignment method (LAGAN). Human gene locations are shown above and human chromosome 16 coordinates below.

Figure 4

Non-synonymous changes along the 1A3/1A4 branch cluster on the SULT1A1 enzyme structure [PDB: 1LS6] [26]. Red sites experienced non-synonymous changes, green sites experienced synonymous changes. The PAPS donor substrate and p-nitrophenol acceptor substrates are shown in blue. Image was generated using Chimera [86].